The invention relates to a method and to an apparatus for the weft insertion in a jet weaving machine where during each work cycle a weft thread of predeterminable length which is to be inserted is drawn off from a winding drum of a thread supply apparatus, the weft thread being inserted into an open shed by means of a main nozzle which is fed with a transport medium, is bound into the cloth through the change of shed, and is severed at the cloth edge which is near the main nozzle.
In jet weaving machines the weft insertion takes place by means of a transport medium which inserts the respective weft thread through the open shed. In air jet weaving machines this transport medium is air.
For the weft insertion a definite and predeterminable thread length, which is naturally dependent on the weaving width, is in each case drawn off from the stationary winding drum of a thread supply apparatus and is supplied to a main nozzle. The main nozzle is fed with compressed air and accelerates the weft thread into the open shed. In particular in the case of air jet weaving machines running at high rotational speeds the main nozzle is typically arranged stationary with respect to the sley, so that the main nozzle carries out the oscillatory movement synchronously with the sley. It is also known, in particular in the case of large weaving widths, to provide a pre-nozzle which is fed with air between the thread supply apparatus and the main nozzle in order to increase the acceleration path for the weft thread. A plurality of auxiliary or relay nozzles, which are likewise fed with compressed air, are usually provided along the weft insertion path. After a successful weft insertion the front end of the weft thread is e.g. taken and held by a stretch nozzle and the weft thread is beat up to the cloth by the sley. Then the change of shed takes place, through which the weft thread is bound in over the entire weaving width. Then the thread must be severed on the insertion side between the main nozzle and the cloth edge which is near it in order to be prepared for the next weft insertion.
Special problems arise in particular when weaving with relatively elastic weft yarns and/or when weaving over large weaving widths of for example four to five meters or more. One problem is that high tensions are present in the thread through the strong draw forces. If now the thread is severed at the insertion side after the change of shed, then it often happens that the severed thread end jumps out backwards, that is, contrary to the weft insertion direction, to the rear out of the main nozzle. This makes a stopping of the weaving machine necessary in order that the end of the thread can be inserted again into the main nozzle. An attempt is admittedly made to solve this problem in that during and after the severing the main nozzle is fed with a retaining air flow, which is intended to hold the thread end in the main nozzle. The result is however unsatisfactory. A lengthening of the nozzle likewise does not lead to a satisfactory result.
A further problem arises in particular through the high acceleration forces which must be exerted by means of the air on the weft thread. At the beginning of the weft insertion there is the danger that the beginning of the weft thread which is accelerated out of the main nozzle does not encounter the entry of the insertion passage. This leads to the formation of knot-like curls or kinks, which as a rule result in stopping of the weaving machine. At the end of the weft insertion the weft thread must be strongly braked. The large deceleration forces which are required for this can lead to so-called stopping or stretching jolts, which means that a tearing of the weft thread arises at the insertion side through the strong braking. The weaving machine must be stopped. In order to avoid this, measures are known in order to brake the weft thread more gently. It is known to deflect the weft thread transversely to the travel or insertion direction respectively prior to the termination of the weft insertion with the help of a pin, so that it forms a loop and as a result travels through an additional path distance. This deflection takes place for example between the pre-nozzle and the main nozzle. If the deflection is made to vanish through pivoting back or drawing back of the pin, then the elastic tension in the thread can be thereby reduced. The deflection of the weft thread however represents a heavy frictional load on the weft yarn. In the case of elastic weft yarns and/or when weaving over large widths the effect which can be achieved is often insufficient.